1. Field of the Invention
The present invention relates to a process for producing a metal material having a displace deposition-plated and doping-modified composite coating layer, and a metal material produced by the process mentioned above and having a displace deposition-plated and doping-modified composite coating layer. More particularly, the present invention relates to a process for producing a metal material which has a displace deposition-plated and doping modified composite coating layer formed by applying a displace deposition-plating procedure onto a surface of a metal base material, for example, aluminum or ferrous base material, and then applying a doping-modifying procedure to the displace deposition-plated coating layer to provide a composite coating layer having an improved adhesion and an enhanced denseness, and a metal material produced by the above-mentioned process, and provided with a displace deposition-plated and doping-modified composite coating layer.
2. Description of the Related Art
Generally, plating processes for forming a metal coating on a target metal material surface by reducing and depositing metal ions contained in a plating solution are classified, in view of the reduction method applied thereto, into the following groups. Namely, the plating processes include electric plating processes in which the reduction is carried out by externally supplying electrons produced by a cathode electrolysis into the plating liquid; chemical plating precesses in which the reduction is effected by adding a reducing agent into a plating liquid; and displace deposition plating processes wherein the plating metal ions are reduced by utilizing an electron emission phenomenon generated due to a dissolution (ionization) of a metal from which the metal base material is formed.
The present invention utilizes a displace deposition plating process. In a displace deposition plating system, when the plating metal exhibits an oxidation-reduction potential (redox potential) not nobler than the oxidation-reduction potential of the metal from which the metal base material is formed, no displace deposition plating occurs.
Namely, the displace deposition plating can be effected by the following reactions; EQU mMeI.fwdarw.mMeI.sup.n+ +mne (Chemical reaction formula 1) EQU nMeII.sup.m+ +mne.fwdarw.nMeII (Chemical reaction formula 2)
In the above-mentioned formulae 1 and 2, MeI represents a metal from which a metal base material to be plated is formed, MeII represents a plating metal, e represents an electron, and m and n represent ionic valences of the metals MeI and MeII ionized in an aqueous plating solution, respectively.
First, when a metal MeI is ionized and dissolved in an amount of m moles in the aqueous solution in accordance with the chemical formula 1, electrons are emitted in the number of mn per m moles of the metal MeI. In this case, when an oxidation-reduction potential of a plating metal MeII is nobler than that of the metal MeII, the plating metal ions MeII.sup.m+ in an amount of n moles in the plating solution receive the electrons in the number of mn and are reduced into metal MeII in accordance with the chemical formula 2, and the resultant metal MeII deposits on the metal base material MeI to form a plated coating layer.
As the above-mentioned mechanism of the displace deposition plating clearly shows, a progress in the coating of the metal base material surface with a plating metal in accordance with the chemical reaction formulae 1 and 2 causes the contact of the metal base material surface with the plating liquid to be difficult and thus the progress in the reaction in accordance with the chemical reaction formula 1 to be obstructed, and then the plating reactions are terminated. This fact shows that, in the displace deposition plating process, the control in the thickness of the plated metal coating layer is very difficult in comparison with that of the electric and chemical plating processes in which the thickness of the target plated coating layer increases substantially proportionally with the plating time.
Where the target plated coating layer is allowed to be relatively thin, the plating reactions in accordance with the chemical reaction formulae 1 and 2 may be stopped before the reactions are completed. However, in this case, the resultant plated coating layer is very rough and has a porous structure, and thus the metal base material surface covered by the plated porous coating layer has a possibility to partially contact with the plating liquid. This plated porous coating layer is disadvantageous in that the mechanical strength and the adhesion with the metal base material surface are unsufficient and the corrosion resistance is poor, and thus is unsatisfactory for practical use.
To solve the above-mentioned problems of the conventional displace deposition plating processes, various attempts were made depending on the combinations of the type of metal from which the metal base material is formed with the type of target plating metal. For example, Japanese Unexamined Patent Publication 2-61,073 discloses a process for plating a copper base plate surface with a tin coating layer having a fine and dense structure at a high speed, and Japanese Unexamined Patent Publication No. 2-185,982 discloses a process for plating a steel base plate surface with a copper coating layer having a high adhesion. In each of the above-mentioned processes, the composition of the plating liquid, especially the additives, is improved to solve the above-mentioned problems. Also, Japanese Unexamined Patent Application No. 3-153,879 discloses a process for plating a tin solder-coating layer having a large thickness on a copper base plate surface, and Japanese Unexamined Patent Publication No. 7-34,254 discloses a process for plating a zinc coating layer having a high adhesion on an aluminum base plate surface. In each of these processes, a pre-treating procedure before the displace deposition plating procedure and a post-treating procedure after the displace deposition plating procedure are improved, to overcome the problems of the conventional processes.
In the above-mentioned processes, however, special means, in response to the combination of the type of the metal base material and the type of the target plating metal, must be taken. Namely, these processes can be utilized only for special purposes but not for general displace deposition plating purposes. Therefore, these prior processes are disadvantageous in that a complicated measure to attain the purpose is necessary, and the plating liquid composition and the plating procedures are complicated. On other hand, in the electric and chemical plating processes, to impart various functions to the plated coating layer, a alloying method, namely a alloy-plating method, and a dispersion plating method in which fine solid particles are dispersed in a plating liquid and co-deposited in a plated coating layer, are carried out.
However, in the displace deposition plating processes, since the oxidation-reduction potential of the plating metal must be nobler than that of the metal base material, there is a limitation to the type of the metals to be alloyed and usable for the displace deposition plating processes. Also, even if the alloying can be effected, among a plurality of the metals, the noblest metal, in oxidation-reduction potential, selectively deposits with top priority, and therefore, such a problem that the composition of the resultant plated coating layer is difficult to control occurs. Further, when the dispersion plating method is applied to the displace deposition plating process, since, as mentioned above, the control in the thickness and mechanical properties of the plated coating layer in the displace deposition plating processes is difficult, the application of the dispersion plating method to the displace deposition plating procedure for forming an outermost functional surface layer is insuitable and no practical use has been reported.
Namely, in almost all of the practical displace deposition plating processes, a single metal is plated and thus the properties of the plated coating layers are limited in practice.
As a case in which the displace deposition plating process must be industrially used, in other words, other plating processes are difficult to practically utilize, only a zincate treatment (displace deposition zinc plating) is known. This zincate treatment is carried out as a pre-treatment for the purpose of enhancing adhesion of a target plating metal to a surface of a aluminum-coating material.
As mentioned above, the conventional displace deposition plating processes have disadvantages to be solved. However, the displace deposition plating processes are industrially advantageous in that plating equipment is very simple and plating cost is low, in comparison with the electric plating processes, because a electricity supply for the plating procedure, which electricity supply is indispensable for the electric plating processes, is unnecessary; and the plating liquid has a simple composition and a high chemical stability in comparison with the chemical plating processes. Therefore, it is very advantageous for industry to improve the displace deposition plating processes and to expand the industrially applicable field thereof.